Scientists Cook Up Saturn Moon Cocktail on Earth

This feature on Titan is at least 100,000 square kilometers (39,000 square miles), which is greater in extent than Lake Superior (82,000 square kilometers or 32,000 square miles), which is one of Earth’s largest lakes.Credit: NASA/JPL/GSFC

The recent
discovery of lakes on Saturn's moon Titan make it the only other object in the
solar system known to have liquid on its surface. However, dipping 179?C
(290?F) below freezing, these lakes are definitely not filled with water.

"The
water is frozen so solid on Titan
that you can liken it to silicate rocks on Earth," says Vincent Chevrier
of the University of Arkansas.

The liquid
on Titan is likely a hydrocarbon cocktail of mostly methane and ethane, judging
from observations by the Cassini-Huygens space probe. The exact proportions are
uncertain because scientists have little data on how these substances behave at
such low temperatures.

"There
was never before much interest in the liquid and solid properties of methane
and ethane, since they are normally gases on Earth's surface," Chevrier
says.

But that's
all changed. Titan's "liquid assets" drive geologic and chemical
processes that may mimic those on our
own planet. To better understand this, Chevrier and his colleagues have
received NASA funding to recreate Titan's surface in a lab.

Lake
country

Thanks to
radar maps taken by Cassini, we know that the polar regions of Titan are dotted
with numerous lakes. Some of these are as large as the Great Lakes in the U.S.

Scientists
are not sure where these large bodies of hydrocarbons come from. One
possibility is that methane
rain and possibly ethane snow drive a "hydrological cycle" that
eventually drains into these lakes.

Alternatively,
Titan may have large reservoirs of liquid underground, and the lakes are the
result of impact craters carving deep enough to expose this sub-surface
ocean.

It has been
difficult to rule out any of these hypotheses in part because the process of
evaporation from the hydrocarbon lakes in Titan?s environment is poorly
understood. If researchers knew how fast the lakes were disappearing, they
would have a better sense of what makes them appear.

"The
rates of exchange of hydrocarbons over seasonal and potentially longer climate
cycles on Titan is an important goal of current research," says Oded
Aharonson of the California Institute of Technology, who is not involved with
this new project.

Mini-lakes

For their
part, Chevrier's team will be measuring the evaporation rates of methane and
ethane in a Titan simulation chamber. To mimic the moon's atmosphere, the
2-meter-high steel cylinder will hold ultra-cold nitrogen gas at a pressure
about 50% higher than on Earth.

Chevrier's
group will introduce small quantities of methane and/or ethane into the
chamber. Below about 95 Kelvin (or ?178 degrees Celsius), the hydrocarbon gas
will condense into roughly 1 centimeter deep "mini-lakes" at the
bottom of the cylinder. The researchers will then raise the temperature
slightly and record the rate of evaporation.

It is
assumed that ethane (whose molecules are heavier than methane's) will have a
much slower evaporation rate, but by how much is unknown. It's even less clear
what happens when methane and ethane are mixed together, along with nitrogen
gas dissolving in from the atmosphere above.

"It is
highly likely that the lakes are in fact complex mixtures of ethane, methane
and nitrogen," Chevrier says. "We will study the behavior of pure
compounds first and then shift to mixtures."

The team
also plans to look at the possibility of other organic
compounds mixing in with the Titan broth and perhaps slowing the
evaporation.

Earth
analog

Determining
the evaporation rates on Titan will not only help sort out the geologic
processes that formed the lakes, it will also provide some needed information
about the atmospheric chemistry.

Titan is
the only moon in our solar system with a substantial atmosphere. The moon?s
surface is totally obscured by an orange haze made up of complex organics
(called tholins), which form when methane is destroyed by ultraviolet light
from the Sun.

This same
sort of organic chemistry may have gotten the biological ball rolling on Earth
billions of years ago.

"Titan
shows how you can have organic reactions without life," Chevrier says.
Such organic chemical reactions may have provided the first necessary steps
towards the origin of life on Earth.

On Titan,
because the methane-fueled organic reactions in the atmosphere end up
destroying the methane molecules, to keep the reactions operating a constant
source of gaseous methane is needed. The evaporation of methane from Titan?s
lakes may be one such source, and Chevrier's data should help say whether it is
enough.

"Titan's
surface is rich with geological features similar to those found on Earth but
based on different materials," says Christophe Sotin from the Jet
Propulsion Lab in Pasadena. "So any lab experiment that can reproduce
conditions on Titan and give some pieces of information of the processes that
can happen on this moon are important."